Positioning and the location-based services (LBS) enabled by accurate positioning determinations are of significant and growing interest in the context of wireless communication networks and their associated subscriber devices, for technical, commercial, and safety-related reasons. Example applications include location-aware social networking applications, advertising applications, and navigation and traffic assistance, along with critical emergency services like E911 in the United States and similar emergency positioning services used elsewhere.
Different services have different accuracy and timing requirements. In many environments, the position of a radio transceiver such as a subscriber terminal can be accurately estimated by using positioning methods based on GPS (Global Positioning System). Many communication networks also provide positioning assistance to subscriber terminals, referred to as User Equipment or UEs in the plural sense. Such positioning assistance enables terminals to perform measurements at lower receiver sensitivity levels and improves the “start up” performance of GPS receivers, referred to as Assisted-GPS positioning or A-GPS.
GPS or A-GPS receivers, however, are not available in all wireless terminals and GPS-based positioning therefore cannot be relied upon in a universal sense with respect to positioning terminals in a wireless communication network. As a further complication, GPS works poorly or not at all in indoor environments and likewise experiences severe problems in urban canyons. A complementary terrestrial positioning method, called Observed Time Difference of Arrival (OTDOA), is therefore being standardized by 3GPP.
With OTDOA, a terminal or other UE measures the timing differences for downlink reference signals received from multiple distinct locations. For each measured neighbor cell, the UE measures Reference Signal Time Difference (RSTD) which is the relative timing difference between signals received from the neighbor cell and the reference cell. The UE position estimate is then found as the intersection of hyperbolas corresponding to the measured RSTDs. At least three measurements from geographically dispersed base stations with a good geometry are needed to solve for two position coordinates of the terminal and the receiver clock bias. Solving for the terminal location requires precise knowledge of the transmitter locations and the corresponding transmit timing offsets.
OTDOA positioning calculations are carried out in the terminals in UE-based positioning solutions, or are carried out in the network with assistance from the UEs in UE-assisted positioning solutions. As an example, the network includes a positioning server that determines UE locations based on RSTD measurements carried out by the UEs and/or the network radio nodes, such as base stations within the network. In Long Term Evolution (LTE) networks, the positioning server may be represented by, e.g., Enhanced Serving Mobile Location Centers (E-SMLCs) or SUPL Location Platforms (SLP) providing positioning services.
To enable positioning in LTE and facilitate positioning measurements of a proper quality and for a sufficient number of distinct locations, new physical signals dedicated for positioning have been introduced. These signals are referred to as positioning reference signals or PRS and low-interference positioning subframes are specified in 3GPP for their transmission, where the low-interference positioning subframes are subframes characterized by, e.g., no or reduced PDSCH transmission activity from radio nodes, to thereby improve the quality of PRS reception and processing at receivers. PRS are transmitted with a pre-defined periodicity of 160, 320, 640 or 1280 ms.
PRS are transmitted from one antenna port—the “R6” port—according to a pre-defined pattern, which may be based on timing. For more information, see 3GPP TS 36.211, Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation. A frequency shift, which is a function of Physical Cell Identity (PCI) can be applied to the specified PRS patterns to generate orthogonal patterns that provide an effective frequency reuse of six. That high reuse factor significantly reduces neighbor cell interference on the measured PRS and thus improves positioning measurements. While positioning measurements could be made for other types of reference signals might be used for positioning, PRS have been specifically designed for positioning measurements and in general are characterized by better signal quality than other reference signals.
Even so, PRS-based positioning measurements can be challenging. For example, for OTDOA-based positioning, the UE measures PRS from multiple distinct locations—e.g., each one of three or more base stations providing service within a group of neighboring cells. With each base station occupying a different location relative to the UE, the PRS from one or more of the base stations may be much weaker than those received by the UE from the UE's serving cell. Furthermore, the UE must perform blind searching unless it has knowledge of at least the approximate arrival time for the PRS and the exact pattern and thus the signal sequence used for PRS transmission.
To ease the search burden, the network can transmit positioning assistance data. Such data includes reference cell information, neighbor cell lists containing PCIs of neighbor cells, the number of consecutive downlink subframes used for PRS transmission within the defined PRS transmission occasions, the transmission bandwidth used for PRS, the frequencies and bands where PRS are transmitted, etc.
As a further limitation, the standard provides for transmission of PRS from a single antenna port, which is at odds with the use of multiple antenna ports for transmission within a given cell. Multiple antenna ports are used, for example, in distributed antenna systems (DAS). Such systems include the case where multiple antennas are coupled to the same feeder cable, systems with remote radio heads (RRH), as well as the more sophisticated example of Coordinated MultiPoint (CoMP) transmission systems in which a number of geographically separated antennas/transmitters are used to provide better coverage within a cell.
At the UE, PRSs transmitted within the same measurement interval are distinguished by the applied PRS pattern. PRS patterns are a function of PCI, and the ability to differentiate between the PRS from different cells thus holds. However, measurement ambiguity would arise at the UE if a given cell simultaneously transmitted the same PRS from more than one physical antenna port at a time, since the UE would not be able to distinguish the signals transmitted simultaneously from more than one antenna port associated with the same PCI
Similar problems arise in relay-augmented cells because the relays may not form their own cells and instead simply provide improved coverage within a donor cell. Thus, to the extent that the relay would retransmit the same PRS as being transmitted by the eNodeB or other base station of the donor cell, measurement ambiguity would arise at UEs simultaneously receiving the same PRS from the relay node and base station. While per-antenna IDs for PRS would solve the ambiguity problem, their use is impractical in view of the number of non-overlapping IDs available in current cell ID schemes, which are not designed specifically for facilitating positioning with PRS.
In contrast to the above PRS ambiguity, CRS may be transmitted from multiple antenna ports. However, measurement quality is typically lower when CRS is used for positioning measurement while being transmitted from more than one port, because CRS transmitted from two antenna ports within a cell, which is a typical case, has an effective frequency reuse of three, whilst PRS have been designed for frequency reuse of six. Further, CRS may be not available in the low-interference subframes intended for PRS, e.g., when Multicast-Broadcast Single Frequency Network (MBSFN) subframes are configured as positioning subframes. Consequently, CRS-based positioning measurements would necessarily need to be performed outside of the low-interference subframes, which degrades the quality of those measurements due to the interference from, e.g., data transmissions.
Here, note that if PRS are permanently not configured for transmission in a cell, then the positioning configuration information element is likely to not be included in the assistance data for the corresponding cell, which excludes the possibility of measuring CRS in low-interference positioning subframes, even when non-MBSFN subframes are configured as positioning subframes.